Configure firewall's kernel exception listener with configured entry point or a default entry point

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | when relying on buggy behaviour
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12581 #12801
| License       | MIT
| Doc PR        | —

The #12296 PR introduced a bug where the firewall's exception listener was sometimes configured with the default entry point returned by `SecurityExtension#createAuthenticationListeners()`. These changes add a regression test and make the configured entry point (if any) always take precedence over a default entry point.

If someone can confirm this fix, it would have my preference merging this over reverting #12296 for Symfony 2.6.1.